1. Field of the Invention
The present invention relates to a machine and method for casting metal parts by directly pouring molten metal from a bottom pour ladle into a mold.
2. Background Art
Molten metal may be cast in molds to form a wide variety of products. Examples of products formed in casting processes include engine heads, engine blocks, transmission housings and a wide variety of other parts. Metal castings may be formed of iron, aluminum or other metals and alloys. Metal casting molds may be sand cast molds, permanent molds or semipermanent molds. Interior cavities may be formed in castings by sand cores or permanent cores. The cores are used to define passages and orifices in a cast metal part and also may be used to lighten the finished cast part.
In casting lines for production casting operations, such as those used to manufacture engine heads, metal to be cast is initially melted in a melting department and then is transferred by means of a transfer ladle to a holding furnace near the casting line. The casting mold is filled by a pouring ladle that transfers molten metal from the holding furnace to a pouring basin on the casting mold. The molten metal flows from the pouring basin through a gating system comprising a network of runners that supply the molten metal to the cavity in the casting mold. The gating system normally supplies molten metal through an ingate in the bottom of the mold cavity. The gating system is designed to avoid turbulence and splashing as the molten metal enters the mold cavity. Turbulence and splashing of the molten metal can cause air to become entrapped in the molten metal as it solidifies, resulting in undesirable porosity in the cast part.
The cost of the metal casting mold is increased by the need to form the gating system. The gating system is made up of a series of passageways extending from the pouring basin to the casting cavity.
One disadvantage of conventional gating systems is as the molten metal flows through the gating system heat is lost to the mold surrounding sprues, runners, and ingates. The molten metal begins to solidify as it cools and, as a result, the metal must be initially heated to a higher temperature to compensate for heat losses as the metal flows through the gating system. Heating metal to a greater extent increases the energy costs that in turn increase the cost of the casting process. Increased energy costs may be incurred both in the initial melting operation and at the holding furnace.
After the casting operation is complete, the metal in the gating system solidifies. The solidified metal in the gating system is mechanically removed from the casting. The yield of a casting operation may be characterized as the ratio of the weight of the rough casting to the poured weight of molten metal. The metal that solidifies in the gating system reduces the yield of the casting operation.
Another disadvantage of the conventional metal casting mold having such a gating system is that inclusions, impurities and oxides may be entrained in the molten metal as it flows through the gating system. The inclusion of oxides in the molten metal may be increased if the molten metal is exposed to oxygen in the in-gate area.
A proposed solution for the above problems is disclosed in U.S. Pat. No. 4,961,460 that discloses a pouring process in which a ceramic sleeve including a filter is provided on top of a sprue that leads into a mold cavity. The sleeve is formed of a ceramic, or refractory, material and supports a ceramic foam filter. The molten metal flows through the filter in a smooth, substantially non-turbulent flow into the mold cavity. One problem with this process is the cost incurred for the refractory sleeve and filter. Difficulties are also encountered relating to retention of the filter in the ceramic sleeve and securing the ceramic sleeve in the mold during the pouring process. The pouring basin requires attachment to the mold when the mold is built and requires periodic replacement. Replacement of the pouring basin further adds to the cost of the process. After each pour, solidified metal in the pouring basin requires re-melting and replacement of the refractory sleeve and filter.
Applicants' invention provides a direct pour casting process that eliminates the need to provide a ceramic pouring basin and filter. Applicants' invention also eliminates the need for extensive gating systems of conventional casting molds and the costs incurred to compensate for thermal losses and other problems associated with conventional casting molds. Applicants' invention is summarized below.